LIGHT

CONCEPT

Light exists along a relatively narrow bandwidth of the electromagnetic spectrum, and the region of visible light is more narrow still. Yet, within that realm are an almost infinite array of hues that quite literally give color to the entire world of human experience. Light, of course, is more than color: it is energy, which travels at incredible speeds throughout the universe. From prehistoric times, humans harnessed light's power through fire, and later, through the invention of illumination devices such as candles and gas lamps. In the late nineteenth century, the first electric-powered forms of light were invented, which created a revolution in human existence. Today, the power of lasers, highly focused beams of high-intensity light, make possible a number of technologies used in everything from surgery to entertainment.

HOW IT WORKS

Early Progress in Understanding of Light

The first useful observations concerning light came from ancient Greece. The Greeks recognized that light travels through air in rays, a term from geometry describing that part of a straight line that extends in one direction only. Upon entering some denser medium, such as glass or water, as Greek scientists noticed, the ray experiences refraction, or bending. Another type of incidence, or contact, between a light ray and any surface, is reflection, whereby a light ray returns, rather than being absorbed at the interface.

The Greeks worked out the basic laws governing reflection and refraction, observing, for instance, that in reflection, the angle of incidence is approximately equal to the angle of reflection. Unfortunately, they also subscribed to the erroneous concept of intromission—the belief that light rays originate in the eye and travel toward objects, making them visible. Some 1,500 years after the high point of Greek civilization, Arab physicist Alhasen (Ibn al-Haytham; c. 965-1039), sometimes called the greatest scientist of the Middle Ages, showed that light comes from a source such as the Sun, and reflects from an object to the eyes.

The next great era of progress in studies of light began with the Renaissance (c. 1300-c. 1600.) However, the most profound scientific achievements in this area belonged not to scientists, but to painters, who were fascinated by color, shading, shadows, and other properties of light. During the early seventeenth century, Galileo Galilei (1564-1642) and German astronomer Johannes Kepler (1571-1630) built the first refracting telescopes, while Dutch physicist and mathematician Willebrord Snell (1580-1626) further refined the laws of refraction.

The Spectrum

Sir Isaac Newton (1642-1727) was as intrigued with light as he was with gravity and the other concepts associated with his work. Though it was not as epochal as his contributions to mechanics, Newton's work in optics, an area of physics that studies the production and propagation of light, was certainly significant.

In Newton's time, physicists understood that a prism could be used for the diffusion of light rays—in particular, to produce an array of colors from a beam of white light. The prevailing belief was that white was a single color like the others, but Newton maintained that it was a combination of all other colors. To prove this, he directed a beam of white light through a prism, then allowed the diffused colors to enter another prism, at which point they recombined as white light.

Newton gave to the array of colors in visible light the term spectrum, (plural, "spectra") meaning the continuous distribution of properties in an ordered arrangement across an unbroken range. The term can be used for any set of characteristics for which there is a gradation, as opposed to an excluded middle. An ordinary light switch provides an example of a situation in which there is an excluded middle: there is nothing between "on" and "off." A dimmer switch, on the other hand, is a spectrum, because a very large number of gradations exist between the two extremes represented by a light switch.

SEVEN COLORS…OR SIX?

The distribution of colors across the spectrum is as follows: red-orange-yellow-green-blue-violet. The reasons for this arrangement, explained below in the context of the electromagnetic spectrum, were unknown to Newton. Not only did he live in an age that had almost no understanding of electromagnetism, but he was also a product of the era called the Enlightenment, when intellectuals (scientists included) viewed the world as a highly rational, ordered mechanism. His Enlightenment viewpoint undoubtedly influenced his interpretation of the spectrum as a set of seven colors, just as there are seven notes on the musical scale.

In addition to the six basic colors listed above, Newton identified a seventh, indigo, between blue and violet. In fact, there is a noticeable band of color between blue and violet, but this is because one color fades into another. With a spectrum, there is a blurring of lines between one color and the next: for instance, orange exists at a certain point along the spectrum, as does yellow, but between them is a nearly unlimited number of orange-yellow and yellow-orange gradations.

Indigo itself is not really a distinct color—just a deep, purplish blue. But its inclusion in the listing of colors on the spectrum has given generations of students a handy mnemonic (memorization) device: the name "ROY G. BIV." These letters form an acrostic (a word constructed from the first letters of other words) for the colors of the spectrum. Incidentally, there is something arbitrary even in the idea of six colors, or for that matter seven musical notes: in both cases, there exists a very large gradation of shades, yet also in both cases, the divisions used were chosen for practical purposes.

Waves, Particles, and Other Questions Concerning Light

THE WAVE-PARTICLE CONTROVERSY BEGINS.

Newton subscribed to the corpuscular theory of light: the idea that light travels as a stream of particles. On the other hand, Dutch physicist and astronomer Christiaan Huygens (1629-1695) maintained that light travels in waves. During the century that followed, adherents of particle theory did intellectual battle with proponents of wave theory. "Battle" is not too strong a word, because the conflict was heated, and had a nationalistic element. Reflecting both the burgeoning awareness of the nation-state among Europeans, as well as Britons' sense of their own island as an entity separate from the European continent, particle theory had its strongest defenders in Newton's homeland, while continental scientists generally accepted wave theory.

According to Huygens, the appearance of the spectrum, as well as the phenomena of reflection and refraction, indicated that light was a wave. Newton responded by furnishing complex mathematical calculations which showed that particles could exhibit the behaviors of reflection and refraction as well. Furthermore, Newton challenged, if light were really a wave, it should be able to bend around corners. Yet, in 1660, an experiment by Italian physicist Francesco Grimaldi (1618-1663) proved that light could do just that. Passing a beam of light through a narrow aperture, or opening, Grimaldi observed a phenomenon called diffraction, or the bending of light.

In view of the nationalistic character that the wave-particle debate assumed, it was ironic that the physicist whose work struck a particularly forceful blow against corpuscular theory was himself an Englishman: Thomas Young (1773-1829), who in 1801 demonstrated interference in light. Directing a light beam through two closely spaced pinholes onto a screen, Young reasoned that if light truly were made of particles, the beams would project two distinct points onto the screen. Instead, what he saw was a pattern of interference—a wave phenomenon.

THE QUESTION OF A MEDIUM.

As the nineteenth century progressed, evidence in favor of wave theory grew. Experiments in 1850 by Jean Bernard Leon Foucault (1819-1868)—famous for his pendulum—showed that light traveled faster in air than through water. Based on studies of wave motion up to that time, Foucault's work added substance to the view of light as a wave.

Foucault also measured the speed of light in a vacuum, a speed which he calculated to within 1% of its value as it is known today: 186,000 mi (299,339 km) per second. An understanding of just how fast light traveled, however, caused a nagging question dating back to the days of Newton and Huygens to resurface: how did light travel?

All types of waves known to that time traveled through some sort of medium: for instance, sound waves were propagated through air, water, or some other type of matter. If light was a wave, as Huygens said, then it, too, must have some medium. Huygens and his followers proposed a weak theory by suggesting the existence of an invisible substance called ether, which existed throughout the universe and which carried light.

Ether, of course, was really no answer at all. There was no evidence that it existed, and to many scientists, it was merely a concept invented to shore up an otherwise convincing argument. Then, in 1872, Scottish physicist James Clerk Maxwell (1831-1879) proposed a solution that must have surprised many scientists. The "medium" through which light travels, Maxwell proposed, was no medium at all; rather, the energy in light is transferred by means of radiation, which requires no medium.

Electromagnetism

Maxwell brought together a number of concepts developed by his predecessors, sorting these out and adding to them. His work led to the identification of a "new" fundamental interaction, in addition to that associated with gravity. This was the mode of particle interaction associated with electromagnetic force.

The particulars of electromagnetic force, waves, and radiation are a subject unto themselves—really, many subjects. As for the electromagnetic spectrum, it is treated at some length in an essay elsewhere in this volume, and the reader is encouraged to review that essay to gain a greater understanding of light and its place in the spectrum.

In addition, some awareness of wave motion and related phenomena would also be of great value, and, for this purpose, other essays are recommended. In the present context, a number of topics relating to these larger subjects will be handled in short order, with a minimum of explanation, to enable a more speedy transition to the subject of principal importance here: light.

ELECTROMAGNETIC WAVES.

There is, of course, no obvious connection between light and the electromagnetic force observed in electrical and magnetic interactions. Yet, light is an example of an electromagnetic wave, and is part of the electromagnetic spectrum. The breakthrough in establishing the electromagnetic quality of light can be attributed both to Maxwell and German physicist Heinrich Rudolf Hertz (1857-1894).

In his Electricity and Magnetism (1873), Maxwell suggested that electromagnetic force might have aspects of a wave phenomenon, and his experiments indicated that electromagnetic waves should travel at exactly the same speed as light. This appeared to be more than just a coincidence, and his findings led him to theorize that the electromagnetic interaction included not only electricity and magnetism, but light as well. Some time later, Hertz proved Maxwell's hypothesis by showing that electromagnetic waves obeyed the same laws of reflection, refraction, and diffraction as light.

Hertz also discovered the photoelectric effect, the process by which certain metals acquire an electrical potential when exposed to light. He could not explain this behavior, and, indeed, there was nothing in wave theory that could account for it. Strangely, after more than a century in which acceptance of wave theory had grown, he had encountered something that apparently supported what Newton had said long before: that light traveled in particles rather than waves.

The wave-Particle Debate Revisited

One of the modern physicists whose name is most closely associated with the subject of light is Albert Einstein (1879-1955). In the course of proving that matter is convertible to energy, as he did with the theory of relativity, Einstein predicted that this could be illustrated by accelerating to speeds close to that of light. (Conversely, he also showed that it is impossible for matter to reach the speed of light, because to do so would—as he proved mathematically—result in the matter acquiring an infinite amount of mass, which, of course, is impossible.)

Much of Einstein's work was influenced by that of German physicist Max Planck (1858-1947), father of quantum theory. Quantum theory and quantum mechanics are, of course, far too complicated to explain in any depth here. It is enough to say that they called into question everything physicists thought they knew, based on Newton's theories of classical mechanics. In particular, quantum mechanics showed that, at the subatomic level, particles behave in ways not just different from, but opposite to, the behavior of larger physical objects in the observable world. When a quantity is "quantized," its values or properties at the atomic or subatomic level are separate from one another—meaning that something can both be one thing and its opposite, depending on how it is viewed.

Interpreting Planck's observations, Einstein in a 1905 paper on the photoelectric effect maintained that light is quantized—that it appears in "bundles" of energy that have characteristics both of waves and of particles. Though light travels in waves, as Einstein showed, these waves sometimes behave as particles, which is the case with the photoelectric effect. Nearly two decades later, American physicist Arthur Holly Compton (1892-1962) confirmed Einstein's findings and gave a name to the "particles" of light: photons.

Light's Place in the Electromagnetic Spectrum

The electromagnetic spectrum is the complete range of electromagnetic waves on a continuous distribution from a very low range of frequencies and energy levels, with a correspondingly long wavelength, to a very high range of frequencies and energy levels, with a correspondingly short wavelength. Included on the electromagnetic spectrum are radio waves and microwaves; infrared, visible, and ultraviolet light; x rays, and gamma rays. As discussed earlier, concerning the visible color spectrum, each of these occupies a definite place on the spectrum, but the divisions between them are not firm: in keeping with the nature of a spectrum, one band simply "blurs" into another.

Of principal concern here is an area near the middle of the electromagnetic spectrum. Actually, the very middle of the spectrum lies within the broad area of infrared light, which has frequencies ranging from 10¹² to just over 10¹⁴ Hz, with wavelengths of approximately 10⁻¹ to 10⁻³ centimeters. Even at this point, the light waves are oscillating at a rate between 1 and 100 trillion times a second, and the wavelengths are from 1 millimeter to 0.01 millimeters. Yet, over the breadth of the electromagnetic spectrum, wavelengths get much shorter, and frequencies much greater.

Infrared lies just below visible light in frequency, which is easy to remember because of the name: red is the lowest in frequency of all the colors, as discussed below. Similarly, ultraviolet lies beyond the highest-frequency color, violet. Neither infrared nor ultraviolet can be seen, yet we experience them as heat. In the case of ultraviolet (UV) light, the rays are so powerful that exposure to even the minuscule levels of UV radiation that enter Earth's atmosphere can cause skin cancer.

Ultraviolet light occupies a much narrower band than infrared, in the area of about 10¹⁵ to 10¹⁶ Hz—in other words, oscillations between 1 and 10 quadrillion times a second. Wavelengths in this region are from just above 10⁻⁶ to about 10⁻⁷ centimeters. These are often measured in terms of a nanometer (nm)—equal to one-millionth of a millimeter—meaning that the wavelength range is from above 100 down to about 10 nm.

Between infrared and ultraviolet light is the region of visible light: the six colors that make up much of the world we know. Each has a specific range and frequency, and together they occupy an extremely narrow band of the electromagnetic spectrum: from 4.3 · 10¹⁴ to 7.5 · 10¹⁴ Hz in frequency, and from 700 down to 400 nm in wavelength. To compare its frequency range to that of the entire spectrum, for instance, is the same as comparing 3.2 to 100 billion.

REAL-LIFE APPLICATIONS

Colors

Unlike many of the topics addressed by physics, color is far from abstract. Numerous expressions in daily life describe the relationship between energy and color: "red hot," for instance, or "blue with cold." In fact, however, red—with a smaller frequency and a longer wavelength than blue—actually has less energy; therefore, blue objects should be hotter.

The phenomenon of the red shift, discovered in 1923 by American astronomer Edwin Hubble (1889-1953), provides a clue to this apparent contradiction. As Hubble observed, the light waves from distant galaxies are shifted to the red end, and he reasoned that this must mean those galaxies are moving away from the Milky Way, the galaxy in which Earth is located.

To generalize from what Hubble observed, when something shows red, it is moving away from the observer. The laws of thermodynamics state that where heat is involved, the movement is always away from an area of high temperature and toward an area of low temperature. Heated molecules that reflect red light are, thus, to use a colloquialism, "showing their tail end" as they move toward an area of low temperature. By contrast, molecules of low temperature reflect bluish or purple light because the tendency of heat is to move toward them.

There are other reasons, aside from heat, that some objects tend to be red and others blue—or another color. Chemical factors may be involved: atoms of neon, for example, can be made to vibrate at a particular wavelength, producing a specific color. In any case, the color that an object reflects is precisely the color that it does not absorb: thus, if something is red, that means it has absorbed every color of the spectrum but red.

WHY IS THE SKY BLUE?

The placement of colors on the electromagnetic spectrum provides an answer to that age-old question posed by generations of children to their parents: "Why is the sky blue?" Electromagnetic radiation is scattered as it enters the atmosphere, but all forms of radiation are not scattered equally. Those having shorter wavelengths—that is, toward the blue end of the spectrum—tend to scatter more than those with longer wavelengths, on the red and orange end.

Yet the longer-wavelength light becomes visible at sunset, when the Sun's light enters the atmosphere at an angle. In addition, the dim quality of evening light means that it is easiest to see light of longer wavelengths. This effect is known as Rayleigh scattering, after English physicist John William Strutt, Lord Rayleigh (1842-1919), who discovered it in 1871. Thanks to Rayleigh's discovery, there is an explanation not only for the question of why the sky is blue, but why sunsets are red, orange, and gold.

RAINBOWS.

On the subject of color as children perceive it, many a child has been fascinated by a rainbow, seeing in them something magical. It is easy to understand why children perceive these beautiful phenomena this way, and why people have invented stories such as that of the pot of gold at the end of a rainbow. In fact, a rainbow, like many other "magical" aspects of daily life, can be explained in terms of physics.

A rainbow, in fact, is simply an illustration of the visible light spectrum. Rain drops perform the role of tiny prisms, dispersing white sunlight, much as scientists before Newton had learned to do. But if there is a pot of gold at the end of the rainbow, it would be impossible to find. In order for a rainbow to be seen, it must be viewed from a specific perspective: the observer must be in a position between the sunlight and the raindrops.

Sunlight strikes raindrops in such a way that they are refracted, then reflected back at an angle so that they represent the entire visible light spectrum. Though they are beautiful to see, rainbows are neither magical nor impossible to reproduce artificially. Such rainbows can be produced, for instance, in the spiral of small water droplets emerging from a water hose, viewed when one's back is to the Sun.

Perception of Light and Color

People literally live and die for colors: the colors of a flag, for instance, present a rallying point for soldiers, and different colors are assigned specific political meanings. Blue, both in the American and French flags, typically stands for liberty. Red can symbolize the blood shed by patriots, or it can mean some version of fraternity or brotherhood. Such is the case with the red of the French tricolor (red-white-blue); likewise, the red in the flag of the former Soviet Union and other Communist countries stood for the alleged international brotherhood of all working peoples. In Islamic countries, by contrast, green stands for the unity of all Muslims.

These are just a few examples, drawn from a specific realm—politics—illustrating the meanings that people ascribe to colors. Similarly, people find meanings in images presented to them by light itself. In his Republic, the ancient Greek philosopher Plato (c. 427-347 b.c.) offered a complex parable, intended to illustrate the difference between reality and illusion, concerning a group of slaves who do not recognize the difference between sunlight and the light of a torch in a cave. Modern writers have noted the similarities between Plato's cave and a phenomenon which the ancient philosopher could hardly have imagined: a movie theatre, in which an artificial light projects images—images that people sometimes perceive as being all too real—onto a screen.

People refer to "tricks of the light," as, for instance, when one seems to see an image in a fire. One particularly well-known "trick of the light," a mirage, is discussed below, but there are also manmade illusions created by light, shapes, and images. An optical illusion is something that produces a false impression in the brain, causing one to believe that something is as it appears, when, in fact, it is not. When two lines of equal length are placed side by side, but one has arrows pointed outward at either end while the other line has arrows pointing inward, it appears that the line with the inward-pointing arrows are shorter.

This is an example of the ways in which human perception plays a role in what people see. That topic, of course, goes far beyond physics and into the realms of psychology and the social sciences. Nonetheless, it is worthwhile to consider, from a physical standpoint, how humans see what they see—and sometimes see things that are not there.

A MIRAGE.

Because they can be demonstrated in light waves as well as in sound waves, diffraction and interference are discussed in separate essays. As for refraction, or the bending of light waves, this phenomenon can be seen in the familiar example of a mirage. While driving down a road on a hot day, one may observe that there are pools of water up ahead, but by the time one approaches them, they disappear.

Of course, the pools were never there; light itself has created an optical illusion of sorts. As light moves from one material to another, it bends with a different angle of refraction, and, though, in this instance, it is traveling entirely through air, it is moving through regions of differing temperature. Light waves travel faster through warm air than through cool air, and, thus, when the light enters the area over the heated surface of the asphalt, it experiences refraction. The waves are thus bent, creating the impression of a reflection, which suggests to the observer that there is water up ahead.

HOW THE EYE SEES COLOR.

White, as noted earlier, is the combination of all colors; black is the absence of color. Where ink, dye, or other forms of artificial pigmentation are concerned, of course, black is a "real" color, but in terms of light, it is not. In the same way, the experience of coldness is real, yet "cold" does not exist as a physical phenomenon: it is simply the absence of heat.

The mixture of pigmentation is an entirely different matter from the mixture of light. In artificial pigmentation, the primary colors—the three colors which, when mixed, yield the remainder of the shades on the rainbow—are red, blue, and yellow. Red mixed with blue creates purple, blue mixed with yellow makes green, and red mixed with yellow yields orange. Black and white are usually created by using natural substances of that color—chalk for white, for instance, or various oxides for black. For light, on the other hand, blue and red are primary colors, but the third primary color is green, not yellow. From these three primary colors, all other shades of the visible spectrum can be made.

The mechanism of the human eye responds to the three primary colors of the visible light spectrum: thus, the eye's retina is equipped with tiny cones that respond to red, blue, and green light. The cones respond to bright light; other structures called rods respond to dim light, and the pupil regulates the amount of light that enters the eye.

The eye responds with maximum sensitivity to light at the middle of the visible color spectrum—specifically, green light with a wavelength of about 555 nm. The optimal wavelength for maximum sensitivity in dim light is around 510 nm, on the blue end. It is difficult for the eye to recognize red light, at the far end of the spectrum, against a dark background. However, this can be an advantage in situations of relative darkness, which is why red light is often used to maintain vision for sailors, amateur astronomers, and the military on night maneuvers. Because there is not much difference between the darkness and the red light, the eye adjusts and is able to see beyond the red light into the darkness. A bright yellow or white light in such situations, on the other hand, would minimize visibility in areas beyond the light.

Artificial Light

PREHISTORIC LIGHTING TECHNOLOGY.

Prehistoric humans did not know it, but they were making use of electromagnetic radiation when they lit and warmed their caves with light from a fire. Though it would seem that warmth was more essential to human survival than artificial light, in fact, it is likely that both functions emerged at about the same time: once humans began using fire for warmth, it would have been a relatively short time before they comprehended the power of fire to drive out both darkness and the fierce creatures (for instance, bears) that came with it.

These distant forebears advanced to the fashioning of portable lighting technology in the form of torches or rudimentary lamps. Torches were probably made by binding together resinous material from trees, while lamps were made either from stones with natural depressions, or from soft rocks—for example, soap-stone or steatite—into which depressions were carved by using harder material. Most of the many hundreds of lamps found by archaeologists at sites in southwestern France are made of either limestone or sandstone. Limestone was a particularly good choice, since it conducts heat poorly; lamps made of sandstone, a good conductor of heat, usually had carved handles to protect the hands of the user.

ARTIFICIAL LIGHT IN PRE-MODERN TIMES.

The history of lighting is generally divided into four periods, each of which overlap, and which together illustrate the slow pace of change in illumination technology. First was the primitive, a period encompassing the torches and lamps of prehistoric human beings—though, in fact, French peasants used the same lighting methods depicted in nearby cave paintings until World War I.

Next came the classical stage, the world of Greece and Rome. Earlier civilizations, such as that of Egypt, belong to the primitive era in lighting—before the relatively widespread adoption of the candle and of vegetable oil as fuel. Third was the medieval stage, which saw the development of metal lamps. Last came the modern or invention stage, which began with the creation of the glass lantern chimney by Leonardo da Vinci (1452-1519) in 1490, culminated with Thomas Edison's (1847-1931) first practical incandescent bulb in 1879, and continues today.

At various times, ancient peoples used the fat of seals, horses, cattle, and fish as fuel for lamps. (Whale oil, by contrast, entered widespread use only during the nineteenth century.) Primitive humans sometimes used entire animals—for example, the storm petrel, a bird heavy in fat—to provide light. Even without such cruel excesses, however, animal fat made for a smoky, dangerous, foul-smelling fire.

The use of vegetable oils, a much more efficient medium for lighting, did not take hold until Greek, and especially, Roman times. Animal oils remained in use, however, among the poor, whose homes often reeked with the odor of castor oil or fish oil. Because virtually all fuels came from edible sources, times of famine usually meant times of darkness as well.

The candle, as well as the use of vegetable oils, dates back to earliest antiquity, but the use of candles only became common among the richest citizens of Rome. Because it used animal fat, the candle was apparently a return to an earlier stage, but its hardened tallow actually represented a much safer, more stable fuel than lamp oil.

INCANDESCENT LIGHT.

Lighting technology in the period from about 1500 to the late nineteenth century involved a number of improvements, but in one respect, little had progressed since prehistoric times: people were still burning fuel to provide illumination. This all changed with the invention of the incandescent bulb, which, though it is credited to Edison, was the product of experimentation that took place throughout the nineteenth century. As early as 1802, British scientist Sir Humphry Davy (1778-1829) showed that electricity running through thin strips of metal could heat them enough to cause them to give off light—that is, electromagnetic radiation.

Edison, in fact, was just one of several inventors in the 1870s attempting to develop a practical incandescent lamp. His innovation lay in his understanding of the parameters necessary for developing such a lamp—in particular, decreasing the electrical resistance in the lamp filament (the part that is heated) so that less energy would be required to light it. On October 19, 1879, using low-resistance filaments of carbon or platinum, combined with a high-resistance carbon filament in a vacuum-sealed glass container, Edison produced the first practical lightbulb.

Much has changed in the design of light-bulbs during the decades following Edison's ingenious invention, of course, but his design provided the foundation. There is just one problem with incandescent light, however—a problem inherent in the definition and derivation of the word incandescent, which comes from a Latin root meaning "to become hot." The efficiency of a light is determined by the ratio of light, or usable energy, to heat—which, except in the case of a campfire, is typically not a desirable form of energy where lighting is concerned.

Amazingly, only about 10% of the energy output from a typical incandescent light bulb is in the form of visible light; the rest comes through the infrared region of the spectrum, producing heat rather than light that people can use. The visible light tends to be in the red and yellow end of the spectrum—closer to infrared—but a blue-tinted bulb helps to absorb some of the red and yellow, providing a color balance. This, however, only further diminishes the total light output, and, hence, in many applications today, fluorescent light takes the place of incandescent light.

Lasers

A laser is an extremely focused, extremely narrow, and extremely powerful beam of light. Actually, the term laser is an acronym, standing for L ight A mplification by S imulated E mission of R adiation. Simulated emission involves bringing a large number of atoms into what is called an "excited state." Generally, most atoms are in a ground state, and are less active in their movements, but the energy source that activates a laser brings about population inversion, a reversal of the ratios, such that the majority of atoms within the active medium are in an excited rather than a ground state. To visualize this, picture a popcorn popper, with the excited atoms being the popping kernels, and the ground-state atoms the ones remaining unpopped. As the atoms become excited, and the excited atoms outnumber the ground ones, they start to cause a multiplication of the resident photons. This is simulated emission.

A laser consists of three components: an optical cavity, an energy source, and an active medium. To continue the popcorn analogy, the "popper" itself—the chamber which holds the laser—is the optical cavity, which, in the case of a laser, involves two mirrors facing one another. One of these mirrors fully reflects light, whereas the other is a partly reflecting mirror. The light not reflected by the second mirror escapes as a highly focused beam. As with the popcorn popper, the power source involves electricity, and the active medium is analogous to the oil in a conventional popper.

TYPES OF LASERS.

There are four types of lasers: solid-state, semiconductor, gas, and dye. Solid-state lasers are generally very large and extremely powerful. Having a crystal or glass housing, they have been implemented in nuclear energy research, and in various areas of industry. Whereas solid-state lasers can be as long as a city block, semiconductor lasers can be smaller than the head of a pin. Semiconductor lasers (involving materials such as arsenic that conduct electricity, but do not do so as efficiently as the metals typically used as conductors) are applied for the intricate work of making compact discs and computer microchips.

Gas lasers contain carbon dioxide or other gases, activated by electricity in much the same way the gas in a neon sign is activated. Among their applications are eye surgery, printing, and scanning. Finally, dye lasers, as their name suggests, use different colored dyes. (Laser light itself, unlike ordinary light, is monochromatic.) Dye lasers can be used for medical research, or for fun—as in the case of laser light shows held at parks in the summertime.

LASER APPLICATIONS.

Laser beams have a number of other useful functions, for instance, the production of compact discs (CDs). Lasers etch information onto a surface, and because of the light beam's qualities, can record far more information in much less space than the old-fashioned ways of producing phonograph records.

Lasers used in the production of CD-ROM (Read-Only Memory) disks are able to condense huge amounts of information—a set of encyclopedias or the New York metropolitan phone book—onto a disk one can hold in the palm of one's hand. Laser etching is also used to create digital videodiscs (DVDs) and holograms. Another way that lasers affect everyday life is in the field of fiber optics, which uses pulses of laser light to send information on glass strands.

Before the advent of fiber-optic communications, telephone calls were relayed on thick bundles of copper wire; with the appearance of this new technology, a glass wire no thicker than a human hair now carries thousands of conversations. Lasers are also used in scanners, such as the price-code checkers at supermarkets and various kinds of tags that prevent thefts of books from libraries or clothing items from stores. In an industrial setting, heating lasers can drill through solid metal, or in an operating room, lasers can remove gallstones or cataracts. Lasers are also used for guiding missiles, and to help building contractors ensure that walls and floors and ceilings are in proper alignment.

WHERE TO LEARN MORE

Burton, Jane and Kim Taylor. The Nature and Science of Colors. Milwaukee, WI: Gareth Stevens Publishing, 1998.

Glover, David. Color and Light. New York: Dorling Kindersley Publishing, 2001.

Kalman, Bobbie and April Fast. Cosmic Light Shows. New York: Crabtree Publishing, 1999.

Kurtus, Ron. "Visible Light" (Web site). <http://www.school-for-champions.com/science/light.html> (May 2, 2001).

"Light Waves and Color." The Physics Classroom (Web site). <http://www.glenbrook.k12.il.us/gbssci/phys/Class/light/lighttoc.html> (May 2, 2001).

Miller-Schroeder, Patricia. The Science and Light of Color. Milwaukee, WI: Gareth Stevens Publishing, 2000.

Nassau, Kurt. Experimenting with Color. New York: F. Watts, 1997.

Riley, Peter D. Light and Color. New York: F. Watts, 1999.

Taylor, Helen Suzanne. A Rainbow Is a Circle: And Other Facts About Color. Brookfield, CT: Copper Beech Books, 1999.

"Visible Light Waves" NASA: National Aeronautics and Space Administration (Web site). <http://imagers.gsfc.nasa.gov/ems/visible.html> (May 2, 2001).

KEY TERMS

APERTURE:

An opening.

DIFFRACTION:

The bending of waves around obstacles, or the spreading of waves by passing them through an aperture.

DIFFUSION:

A process by which the concentration or density of something isdecreased.

ELECTROMAGNETIC SPECTRUM:

The complete range of electromagnetic waves on a continuous distribution from a very low range of frequencies and energylevels, with a correspondingly long wavelength, to a very high range of frequencies and energy levels, with a correspondingly short wavelength. Included on the electromagnetic spectrum are long-wave and short-wave radio; microwaves; infrared, visible, and ultraviolet light; x rays, and gamma rays.

ELECTROMAGNETIC WAVE:

A transverse wave with electric and magnetic fields that emanate from it. The directions of these fields are perpendicular to one another, and both are perpendicular to the line of propagation for the wave itself.

FREQUENCY:

The number of waves passing through a given point during the interval of one second. The higher the frequency, the shorter the wavelength.

HERTZ:

A unit for measuring frequency, named after nineteenth—century German physicist Heinrich Rudolf Hertz (1857-1894).

INCIDENCE:

Contact between a ray—for example, a light ray—and a surface. Types of incidence include reflection and refraction.

MEDIUM:

A substance through which light travels, such as air, water, or glass. Because light moves by radiation, it does not require a medium, and, in fact, movement through a medium slows the speed of light somewhat.

OPTICS:

An area of physics that studies the production and propagation of light.

PHOTOELECTRIC EFFECT:

The phenomenon whereby certain metalsacquire an electrical potential when exposed to light.

PHOTON:

A particle of electromagnetic radiation—for example, light—carryinga specific amount of energy, measured in electron volts (eV).

PRISM:

A three-dimensional glassshape used for the diffusion of light rays.

PROPAGATION:

The act or state of traveling from one place to another.

RADIATION:

The transfer of energy by means of electromagnetic waves, which require no physical medium (for example, water or air) for the transfer. Earth receives the Sun's energy (including its light), via the electromagnetic spectrum, by means of radiation.

RAY:

In geometry, a ray is that part of a straight line that extends in one directiononly. The term "ray" is used to describe the directed line made by light as it moves through space.

REFLECTION:

A type of incidence whereby a light ray is returned toward its source rather than being absorbed at the interface.

REFRACTION:

The bending of a lightray that occurs when it passes through a dense medium, such as water or glass.

SPECTRUM:

The continuous distribution of properties in an ordered arrangement across an unbroken range. Examples of spectra (the plural of "spectrum") include the colors of visible light, or the electromagnetic spectrum of which visiblelight is a part.

TRANSVERSE WAVE:

A wave in which the vibration or motion is perpendicular to the direction in which the wave is moving.

VACUUM:

An area of space devoid of matter, including air.

WAVELENGTH:

The distance between a crest and the adjacent crest, or the trough and an adjacent trough, of a wave. The shorter the wavelength, the higher the frequency.

light¹ / līt/ • n. 1. the natural agent that stimulates sight and makes things visible: the light of the sun [in sing.] the street lamps shed a faint light into the room. ∎  a source of illumination, esp. an electric lamp: a light came on in his room. ∎  (lights) decorative illuminations: Christmas lights. ∎  a traffic light: turn right at the light. ∎  [in sing.] an expression in someone's eyes indicating a particular emotion or mood: a shrewd light entered his eyes. ∎  the amount or quality of light in a place: the plant requires good light in some lights she could look beautiful. 2. understanding of a problem or mystery; enlightenment: she saw light dawn on the woman's face. ∎  spiritual illumination by divine truth. ∎  (lights) a person's opinions, standards, and abilities: leaving the police to do the job according to their lights.3. an area of something that is brighter or paler than its surroundings: sunshine will brighten the natural lights in your hair.4. a match or lighter that produces a flame or spark. ∎  the flame produced: he asked me for a light.5. a window or opening in a wall to let light in. ∎  any of the perpendicular divisions of a mullioned window. ∎  any of the panes of glass forming the roof or side of a greenhouse or the top of a cold frame.6. a person notable or eminent in a particular sphere of activity or place: such lights of Liberalism as the historian Goldwin Smith. • v. (past and past part. lit / lit/ or light·ed) [tr.] 1. provide with light or lighting; illuminate: the room was lighted by a number of small lamps lightning suddenly lit up the house. ∎  switch on (an electric light): only one of the table lamps was lit. ∎  [intr.] (light up) become illuminated: the sign to fasten seat belts lit up.2. make (something) start burning; ignite: Allen gathered sticks and lit a fire [as adj.] (lighted or lit) a lighted cigarette. ∎  [intr.] begin to burn; be ignited: the gas wouldn't light properly. ∎  (light something up) ignite a cigarette, cigar, or pipe and begin to smoke it: she lit up a cigarette and puffed on it serenely [intr.] workers who light up in prohibited areas face dismissal. • adj. 1. having a considerable or sufficient amount of natural light; not dark: the bedrooms are light and airy it was almost light outside.2. (of a color) pale: her eyes were light blue.PHRASES: bring (or come) to light make (or become) widely known or evident: an investigation to bring to light examples of extravagant expenditure.go out like a light inf. fall asleep or lose consciousness suddenly.in a —— light in the way specified; so as to give a specified impression: the audit portrayed the company in a very favorable light.in (the) light of drawing knowledge or information from; taking (something) into consideration: the exorbitant prices are explainable in the light of the facts.light a fire under someonesee fire.light at the end of the tunnel a long-awaited indication that a period of hardship or adversity is nearing an end.light the fusesee fuse² .the light of day daylight. ∎  general public attention: bringing old family secrets into the light of day.the light of someone's life a much loved person.lights out bedtime in a school dormitory, military barracks, or other institution, when lights should be switched off. ∎  a bell, bugle call, or other signal announcing this.lit up inf., dated drunk.see the light understand or realize something after prolonged thought or doubt. ∎  undergo religious conversion.see the light of day be born. ∎ fig. come into existence; be made public, visible, or available: this software first saw the light of day back in 1993.shed (or throw or cast) light on help to explain (something) by providing further information about it.PHRASAL VERBS: light up (or light something up) (with reference to a person's face or eyes) suddenly become or cause to be animated with liveliness or joy: his eyes lit up and he smiled a smile of delight lit up her face.DERIVATIVES: light·ish adj. light·less adj.light·ness n.ORIGIN: Old English lēoht, līht (noun and adjective), līhtan (verb), of Germanic origin; related to Dutch licht and German Licht, from an Indo-European root shared by Greek leukos ‘white’ and Latin lux ‘light.’light² • adj. 1. of little weight; easy to lift: they are very light and portable you're as light as a feather. ∎  deficient in weight, esp. by a specified amount: the sack of potatoes is 5 pounds light. ∎  not strongly or heavily built or constructed; small of its kind: light, impractical clothes light armor. ∎  carrying or suitable for small loads: light commercial vehicles. ∎  carrying only light armaments: light infantry. ∎  (of a vehicle, ship, or aircraft) traveling unladen or with less than a full load. ∎  (of food or a meal) small in quantity and easy to digest: a light supper. ∎  (of a foodstuff) low in fat, cholesterol, sugar, or other rich ingredients: stick to a light diet. ∎  (of drink) not too sweet or rich in flavor or strongly alcoholic: a glass of light Hungarian wine. ∎  (of food, esp. pastry or sponge cake) fluffy or well aerated during cooking. ∎  (of soil) friable, porous, and workable. ∎  (of an isotope) having not more than the usual mass; (of a compound) containing such an isotope.2. relatively low in density, amount, or intensity: passenger traffic was light light summer breezes trading was light for most of the day. ∎  (of sleep or a sleeper) easily disturbed. ∎  easily borne or done: he received a relatively light sentence some light housework.3. gentle or delicate: she planted a light kiss on his cheek my breathing was steady and light. ∎  (of a building) having an appearance suggestive of lightness: the building is lofty and light in its tall nave and choir. ∎  (of type) having thin strokes; not bold.4. (of entertainment) requiring little mental effort; not profound or serious: pop is thought of as light entertainment some light reading. ∎  not serious or solemn: his tone was light. ∎  free from worry or unhappiness; cheerful: I left the island with a light heart.5. archaic (of a woman) unchaste; promiscuous.PHRASES: be light on be rather short of: light on hard news.be light on one's feet (of a person) be quick or nimble.a (or someone's) light touch the ability to deal with something delicately, tactfully, or in an understated way: a novel that handles its tricky subject with a light touch.make light of treat as unimportant: I didn't mean to make light of your problems.make light work of accomplish (a task) quickly and easily.travel light travel with a minimum load or minimum luggage.DERIVATIVES: light·ish adj. light·ly adv.light·ness n.light³ • v. (past and past part. lit / lit/ or light·ed) [intr.] 1. (light on/upon) come upon or discover by chance: he lit on a possible solution.2. archaic descend: from the horse he lit down. ∎  (light on) fall and settle or land on (a surface): a feather just lighted on the ground.PHRASAL VERBS: light into inf. criticize severely; attack: he lit into him for his indiscretion.light out inf. depart hurriedly.

Light

Resources

Light can be narrowly defined as the visible portion of the electromagnetic spectrum. It is a type of energy, and it is made up of particles called photons. In a vacuum such as in space, light moves at 186,000 miles per second (300,000 kilometers per second), or what is called the speed of light. A broader definition would include infrared, ultraviolet, and x-ray wavelengths, which are not visible to the eye. The nature of light has been the subject of controversy for thousands of years. Even today, while scientists know how light behaves, they do not always know why light behaves as it does.

The Greeks were the first to theorize about the nature of light. Led by the scientists Euclid and Hero (first century AD), they came to recognize that light traveled in a straight line. However, they believed that vision worked by intromission—that is, that light rays originated at the eye and traveled to the object being seen. Despite this erroneous hypothesis, the Greeks were able to successfully study the phenomena of reflection and refraction and derive the laws governing them. In reflection, they learned that the angles of incidence and reflection were approximately equal; in refraction, they saw that a beam of light would bend as it entered a denser medium (such as water or glass) and bend back the same amount as it exited.

The next contributor to the embryonic science of optics was Arab mathematician and physicist Alhazen (965–1039), who is sometimes called the greatest scientist of the Middle Ages. Experimenting around the year 1000, he showed that light comes from a source (the sun) and reflects from an object to the eyes, thus allowing the object to be seen. He also studied mirrors and lenses and further refined the laws of reflection and refraction.

By the twelfth century, scientists felt they had solved the riddles of light and color. English philosopher Francis Bacon (1561–1626) contended that light was a disturbance in an invisible medium which could be detected by the eye; subsequently, color was caused by objects “staining” the light as it passed. More productive research into the behavior of light was sparked by the new class of realistic painters, who strove to better understand perspective and shading by studying light and its properties.

In the early 1600s, the refracting telescope was perfected by Galileo and Johannes Kepler, providing a reliable example of the laws of refraction. These laws were further refined by Willebrord Snel, whose name is most often associated with the equations for determining the refraction of light. By the mid-1600s, enough was known about the behavior of light to allow for the formulation of a wide range of theories.

English physicist and mathematician Sir Isaac Newton (1642–1727) was intrigued by the so-called “phenomenon of colors”—the ability of a prism to produce colors from white light. It had been generally accepted that white was a single color, and that a prism could somehow combine white light with others to form a multicolored mixture. Newton, however, doubted this assumption. He used a second prism to recombine the rainbow spectrum back into a beam of white light; this showed that white light must be a combination of colors, not the other way around.

Newton performed his experiments in 1666 and announced them shortly thereafter, subscribing to the corpuscular (or particulate) theory of light. According to this theory, light travels as a stream of particles that originate from a bright source and are absorbed by the eye. Aided by Newton’s reputation, the corpuscular theory soon became accepted throughout Great Britain and in parts of Europe.

In the European scientific community, many scientists believed that light, like sound, traveled in waves. This group of scientists was most successfully represented by Dutch physicist Christiaan Huygens (1629–1695), who challenged Newton’s corpuscular theory. He argued that a wave theory could best explain the appearance of a spectrum as well as the phenomena of reflection and refraction.

Newton immediately attacked the wave theory. Using some complex calculations, he showed that particles, too, would obey the laws of reflection and refraction. He also pointed out that, if truly a wave form, light should be able to bend around corners, just as sound does; instead it cast a sharp shadow, further supporting the corpuscular theory.

In 1660, however, Italian mathematician and physicist Francesco Maria Grimaldi (1618–1663) examined a beam of light passing through a narrow slit. As it exited and was projected upon a screen, faint fringes could be seen near the edge. This seemed to indicate that light did bend slightly around corners; the effect, called diffraction, was adopted by Huygens and other theorists as further proof of the wave nature of light.

One piece of the wave theory remained unexplained. At that time, all known waves moved through some kind of medium—for example, sound waves moved through air and kinetic waves moved through water. Huygens and his allies had not been able to show just what medium light waves moved through; instead, they contended that an invisible substance called ether filled the universe and allowed the passage of light. This unproven explanation did not earn further support for the wave theory, and the Newtonian view of light prevailed for more than a century.

The first real challenge to Newton’s corpuscular theory came in 1801, when English physicist and physician Thomas Young (1773–1829) discovered interference in light. He passed a beam of light through two closely spaced pinholes and onto a screen. If light were truly particulate, Young argued, the holes would emit two distinct streams that would appear on the screen as two bright points. What was projected on the screen instead was a series of bright and dark lines—an interference pattern typical of how waves would behave under similar conditions.

If light is a wave, then every point on that wave is potentially a new wave source. As the light passes through the pinholes it exits as two new wave fronts, which spread out as they travel. Because the holes are placed close together, the two waves interact. In some places the two waves combine (constructive interference), whereas in others they cancel each other out (destructive interference), thus producing the pattern of bright and dark lines. Such interference had previously been observed in both water waves and sound waves and seemed to indicate that light, too, moved in waves.

The corpuscular view did not die easily. Many scientists had allied themselves with the Newtonian theory and they were unwilling to risk their reputations to support an antiquated wave theory. Also, English scientists were not pleased to see one of their countrymen challenge the theories of Newton; Young, therefore, earned little favor in his homeland.

Throughout Europe, however, support for the wave nature of light continued to grow. In France, Etienne-Louis Malus (1775–1826) and Augustin Jean Fresnel (1788–1827) experimented with polarized light, an effect that could only occur if light acted as a transverse wave (a wave which oscillated at right angles to its path of travel). In Germany, Joseph von Fraunhofer (1787–1826) was constructing instruments to better examine the phenomenon of diffraction and succeeded in identifying within the sun’s spectrum 574 dark lines corresponding to different wavelengths.

In 1850 two French scientists, Ján Foucault and Armand Fizeau, independently conducted an experiment that would strike a serious blow to the corpuscular theory of light. An instructor of theirs, Dominique-Françios Arago, had suggested that they attempt to measure the speed of light as it traveled through both air and water. If light were particulate it should move faster in water; if, on the other hand, it were a wave it should move faster in air. The two scientists performed their experiments, and each came to the same conclusion: light traveled more quickly through air and was slowed by water.

Even as more and more scientists subscribed to the wave theory, one question remained unanswered: through what medium did light travel? The existence of ether had never been proven—in fact, the very idea of it seemed ridiculous to most scientists. In 1872, Scottish physicist James Clerk Maxwell (1831–1879) suggested that waves composed of electric and magnetic fields could propagate in a vacuum, independent of any medium. This hypothesis was later proven by German physicist Heinrich Rudolph Hertz (1857–1894), who showed that such waves would also obey all the laws of reflection, refraction, and diffraction. It became generally accepted that light acted as an electromagnetic wave.

Hertz, however, had also discovered the photoelectric effect, by which certain metals would produce an electrical potential when exposed to light. As scientists studied the photoelectric effect, it became clear that a wave theory could not account for this behavior; in fact, the effect seemed to indicate the presence of particles. For the first time in more than a century there was new support for Newton’s corpuscular theory of light.

The photoelectric effect was explained by German–American physicist Albert Einstein (1879–1955) in 1905 using the principles of quantum physics developed by Max Planck. Einstein claimed that light was quantized—that is, it appeared in “bundles” of energy. While these bundles traveled in waves, certain reactions (like the photoelectric effect) revealed their particulate nature. This theory was further supported in 1923 by American physicist Arthur Holly Compton (1892–1962), who showed that the bundles of light—which he called photons—would sometimes strike electrons during scattering, causing their wavelengths to change.

By employing the quantum theories of Planck and Einstein, Compton was able to describe light as both a particle and a wave, depending upon the way it was tested. While this may seem paradoxical, it remains an acceptable model for explaining the phenomena associated with light and is the dominant theory of the 2000s.

See also Photon.

Resources

BOOKS

Brooker, Geoffrey. Modern Classical Optics. Oxford, UK: Oxford University Press, 2003.

Chartier, Germain. Introduction to Optics. New York: Springer, 2005.

Menn, Naftaly. Practical Optics. Amsterdam, Netherlands, and Boston, MA: Elsevier Academic Press, 2004.

Weiner, John. Light-matter Interaction. Hoboken, NJ: Wiley, 2003.

Light

Light is generally defined as that portion of the electromagnetic spectrum with wavelengths between 400 and 700 nanometers (billionths of a meter). Like all forms of electromagnetic radiation, light travels with a speed of 186,282 miles (299,728 kilometers) per second in a vacuum. It is perhaps the swiftest and most delicate form of energy found in nature.

Historical concepts

Considering how important light is in our daily lives, it is hardly surprising that philosophers and scientists have been trying to understand its fundamental nature for centuries. The ancient Greeks, for example, worked out some of the basic laws involving light, including the laws of reflection (bouncing off an object) and refraction (bending through an object). They did so in spite of the fact that they started with only an incorrect concept of light. They believed that light beams started out in the human eye and traveled to an object.

Words to Know

Corpuscle: A particle.

Diffraction: The bending of light or another form of electromagnetic radiation as it passes through a tiny hole or around a sharp edge.

Duality: The tendency of something to behave in two very different ways, for example, as both energy and matter.

Electromagnetic spectrum: The whole range of radiation that travels through a vacuum with a speed of about 3 × 108 meters per second.

Ether: Also spelled aether; medium that was hypothesized by physicists to explain the wave behavior of light.

Photoelectric effect: The production of an electric current when a beam of light is shined on a metal.

Photon: A tiny package of light energy.

Wave: A regular pattern of motion that involves some kind of disturbance in a medium.

Wavelength: The distance between two successive identical parts of a wave, such as two crests or two troughs.

With the rise of modern physics in the seventeenth century, scientists argued over two fundamental explanations of the nature of light: wave versus particle. According to the particle theory of light, light consists of a stream of particles that come from a source (such as the Sun or a lamp), travel to an object, and are then reflected to an observer. This view of light was first proposed in some detail by Isaac Newton (1642–1727). Newton's theory is sometimes known as the corpuscular theory of light.

At about the same time, the wave theory of light was being developed. According to the wave theory, light travels through space in the form of a wave, similar in some ways to water waves. The primary spokesperson for this concept was Dutch physicist Christiaan Huygens (1629–1695).

Over time, the wave theory became more popular among physicists. One of the main reasons for the triumph of the wave theory was that many typical wave properties were detected for light. For example, when light passes through a tiny pinhole or around a sharp edge, it exhibits a property known as diffraction. Diffraction is well known as a property of waves among physicists. Almost anyone can witness the diffraction of water waves as they enter a bay or harbor, for example. If light exhibits diffraction, scientists thought, then it must be transmitted by waves.

Today, scientists usually talk about light as if it were transmitted by waves. They talk about the wavelength and frequency of light, both properties of waves, not particles.

The mysterious ether

One of the serious problems arising out of the wave theory of light is the problem of medium. Wave motion is the regular up-and-down motion of some material. For water waves, that material (or medium) is water. If light is a form of wave motion, scientists asked, what is the medium through which it travels?

The obvious answer, of course, is that light travels through air as a medium. But that answer is contradicted by the fact that light also travels through a vacuum, a region of space that contains no air or anything else.

To resolve this problem, scientists developed the concept of an ether (or aether). The ether was defined as a very thin material—perhaps like air, but much less dense—that permeates all of space. Light could be explained, then, as a wave motion in the ether.

Unfortunately, efforts to locate the ether were unsuccessful. In one of the most famous negative experiments of all time, two American physicists, Albert A. Michelson (1852–1931) and Edward W. Morley (1838–1923), devised a very precise method for detecting the ether. No matter how carefully they searched, they found no ether. Their experiments were so carefully designed and carried out that physicists were convinced that the ether did not exist.

Today, a somewhat simpler view of light as a wave phenomenon exists. Light is a form of radiation that needs no medium through which to travel. It consists of electric and magnetic fields that pulsate up and down as they travel through space.

Return of the corpuscular theory

By the early 1900s, most physicists had accepted the idea that light is a form of wave motion. But they did so somewhat reluctantly because some facts about light could not really be explained by the wave theory. The most important of these was the photoelectric effect.

The photoelectric effect was first observed by German physicist Heinrich Hertz (1857–1894) in about 1888. He noticed that when light is shined on a piece of metal, an electric current (a flow of electrons) is produced. Later experiments showed a rather peculiar property of the photoelectric effect. It doesn't make any difference how intense the light is that is shined on the metal. A bright light and a dim light both produce the same current. What does make a difference is the color of the light. Red light, for example, produces more of a current than blue light.

Unfortunately, there is no way for the wave theory of light to explain this effect. In fact, it was not until 1905 that a satisfactory explanation of the photoelectric effect was announced. That explanation came from German-born American physicist Albert Einstein (1879–1955). Einstein showed that the photoelectric effect could be explained provided that light were thought of not as a wave but as a bundle of tiny particles.

But the concept of light-as-particles is just what Isaac Newton had proposed more than 200 years earlier—and what physicists had largely rejected. The important point about Einstein's explanation, however, was that it worked. It explained a property of light that wave theory could not explain.

Duality of light and matter

The conflict between light-as-waves and light-as-particles has had an interesting resolution. Today, physicists say that light sometimes acts like a wave and sometimes acts like a collection of particles. Perhaps it is a wave consisting of tiny particles. Those particles are now called photons. They are different from other kinds of particles we know of since they have no mass. They are just tiny packages of energy that act like particles of matter.

Two sets of laws are used to describe light. One set is based on the idea that light is a wave. Those laws are used when they work. The second set is based on the idea that light consists of particles. Those laws are also used when they work.

The philosophy of using wave or particle explanations for light is an example of duality. The term duality means that some natural phenomenon can be understood in two very different ways. Interestingly enough, other forms of duality have been discovered. For example, scientists have traditionally thought of electrons as a form of matter. They have mass and charge, which are characteristics of matter. But it happens that some properties of electrons can best be explained if they are thought of as waves. So, like light, electrons also have a dual character.

Čerenkov Effect

The Čerenkov effect (pronounced che-REN-kof) is the emission of light from something transparent when a charged particle travels through the material with a speed faster than the speed of light in that material. The effect is named for Russian physicist Pavel A. Čerenkov (1904–1990), who first observed it in 1934.

Many people have seen the Čerenkov effect without realizing it. In photographs of a nuclear power plant, the water surrounding the reactor core often seems to glow with an eery blue light. That light is Čerenkov radiation produced when rapidly moving particles produced in the core travel through the cooling water around it.

The definition of the Čerenkov effect often puzzles students because it includes references to charged particles traveling faster than the speed of light. Of course, nothing can travel faster than the speed of light in a vacuum. In a fluid such as air, water, plastic, or glass, however, it is possible for objects to travel faster than the speed of light. When they do so, they produce the bluish glow seen in a nuclear reactor.

Making light stand still

In January 2001, scientists at two separate laboratories in Cambridge, Massachusetts, conducted landmark experiments in which they brought light particles to a halt and then sped them back up to their normal speed. In the experiments, the scientists created chambers that held a gas. One research team used sodium gas, the other used the gas form of rubidium, an alkaline metal element. The gases in both chambers were chilled magnetically to within a few millionths of a degree of absolute zero, or −459°F (−273°C). The scientists passed a light beam into the specially prepared chambers, and the light became fainter and fainter as it slowed and then eventually stopped. Even thought the light vanished, the information on its particles was still imprinted on the atoms of sodium and rubidium. That information was basically frozen or stored. The scientists then flashed a second light through the gas, which essentially reconstituted or revived the original beam. The light left each of the chambers with almost the same shape, intensity, and other properties it had when it entered the chambers.

Scientists believe the biggest impact of these experiments could come in futuristic technologies such as ultra-fast quantum computers. The light could be made to carry so-called quantum information, which involves particles that can exist in many places or states at once. Computers employing such technology could run through operations vastly faster than existing machines.

[See also Electromagnetic spectrum; Interference; Photoelectric effect; Wave motion ]

Light

Light can be narrowly defined as the visible portion of the electromagnetic spectrum . A broader definition would include infrared, ultraviolet, and x-ray wavelengths, which are not visible to the eye . The nature of light has been the subject of controversy for thousands of years. Even today, while scientists know how light behaves, they do not always know why light behaves as it does.

The Greeks were the first to theorize about the nature of light. Led by the scientists Euclid and Hero (first century a.d.), they came to recognize that light traveled in a straight line. However, they believed that vision worked by intromission—that is, that light rays originated at the eye and traveled to the object being seen. Despite this erroneous hypothesis, the Greeks were able to successfully study the phenomena of reflection and refraction and derive the laws governing them. In reflection, they learned that the angles of incidence and reflection were approximately equal; in refraction, they saw that a beam of light would bend as it entered a denser medium (such as water or glass ) and bend back the same amount as it exited.

The next contributor to the embryonic science of optics was the Arab mathematician and physicist Alhazen (965-1039), who is sometimes called the greatest scientist of the Middle Ages. Experimenting around the year 1000, he showed that light comes from a source (the Sun ) and reflects from an object to the eyes, thus allowing the object to be seen. He also studied mirrors and lenses and further refined the laws of reflection and refraction.

By the twelfth century, scientists felt they had solved the riddles of light and color . The English philosopher Francis Bacon (1561-1626) contended that light was a disturbance in an invisible medium which could be detected by the eye; subsequently, color was caused by objects "staining" the light as it passed. More productive research into the behavior of light was sparked by the new class of realistic painters, who strove to better understand perspective and shading by studying light and its properties.

In the early 1600s, the refracting telescope was perfected by Galileo and Johannes Kepler, providing a reliable example of the laws of refraction. These laws were further refined by Willebrord Snel, whose name is most often associated with the equations for determining the refraction of light. By the mid-1600s, enough was known about the behavior of light to allow for the formulation of a wide range of theories.

The renowned English physicist and mathematician Isaac Newton was intrigued by the so-called "phenomenon of colors"—the ability of a prism to produce colors from white light. It had been generally accepted that white was a single color, and that a prism could somehow combine white light with others to form a multicolored mixture. Newton, however, doubted this assumption. He used a second prism to recombine the rainbow spectrum back into a beam of white light; this showed that white light must be a combination of colors, not the other way around.

Newton performed his experiments in 1666 and announced them shortly thereafter, subscribing to the corpuscular (or particulate) theory of light. According to this theory, light travels as a stream of particles that originate from a bright source and are absorbed by the eye. Aided by Newton's reputation, the corpuscular theory soon became accepted throughout Great Britain and in parts of Europe .

In the European scientific community, many scientists believed that light, like sound, traveled in waves. This group of scientists was most successfully represented by the Dutch physicist Christiaan Huygens, who challenged Newton's corpuscular theory. He argued that a wave theory could best explain the appearance of a spectrum as well as the phenomena of reflection and refraction.

Newton immediately attacked the wave theory. Using some complex calculations, he showed that particles, too, would obey the laws of reflection and refraction. He also pointed out that, if truly a wave form, light should be able to bend around corners, just as sound does; instead it cast a sharp shadow, further supporting the corpuscular theory.

In 1660, however, Francesco Grimaldi examined a beam of light passing through a narrow slit. As it exited and was projected upon a screen, faint fringes could be seen near the edge. This seemed to indicate that light did bend slightly around corners; the effect, called diffraction , was adopted by Huygens and other theorists as further proof of the wave nature of light.

One piece of the wave theory remained unexplained. At that time, all known waves moved through some kind of medium—for example, sound waves moved through air and kinetic waves moved through water. Huygens and his allies had not been able to show just what medium light waves moved through; instead, they contended that an invisible substance called ether filled the universe and allowed the passage of light. This unproven explanation did not earn further support for the wave theory, and the Newtonian view of light prevailed for more than a century.

The first real challenge to Newton's corpuscular theory came in 1801, when English physicist Thomas Young discovered interference in light. He passed a beam of light through two closely spaced pinholes and onto a screen. If light were truly particulate, Young argued, the holes would emit two distinct streams that would appear on the screen as two bright points. What was projected on the screen instead was a series of bright and dark lines—an interference pattern typical of how waves would behave under similar conditions.

If light is a wave, then every point on that wave is potentially a new wave source. As the light passes through the pinholes it exits as two new wave fronts, which spread out as they travel. Because the holes are placed close together, the two waves interact. In some places the two waves combine (constructive interference), whereas in others they cancel each other out (destructive interference), thus producing the pattern of bright and dark lines. Such interference had previously been observed in both water waves and sound waves and seemed to indicate that light, too, moved in waves.

The corpuscular view did not die easily. Many scientists had allied themselves with the Newtonian theory and were unwilling to risk their reputations to support an antiquated wave theory. Also, English scientists were not pleased to see one of their countrymen challenge the theories of Newton; Young, therefore, earned little favor in his homeland.

Throughout Europe, however, support for the wave nature of light continued to grow. In France, Etienne-Louis Malus (1775-1826) and Augustin Jean Fresnel (1788-1827) experimented with polarized light, an effect that could only occur if light acted as a transverse wave (a wave which oscillated at right angles to its path of travel). In Germany, Joseph von Fraunhofer (1787-1826) was constructing instruments to better examine the phenomenon of diffraction and succeeded in identifying within the Sun's spectrum 574 dark lines corresponding to different wavelengths.

In 1850 two French scientists, Jéan Foucault and Armand Fizeau, independently conducted an experiment that would strike a serious blow to the corpuscular theory of light. An instructor of theirs, Dominique-Françios Arago, had suggested that they attempt to measure the speed of light as it traveled through both air and water. If light were particulate it should move faster in water; if, on the other hand, it were a wave it should move faster in air. The two scientists performed their experiments, and each came to the same conclusion: light traveled more quickly through air and was slowed by water.

Even as more and more scientists subscribed to the wave theory, one question remained unanswered: through what medium did light travel? The existence of ether had never been proven—in fact, the very idea of it seemed ridiculous to most scientists. In 1872, James Clerk Maxwell suggested that waves composed of electric and magnetic fields could propagate in a vacuum , independent of any medium. This hypothesis was later proven by Heinrich Rudolph Hertz, who showed that such waves would also obey all the laws of reflection, refraction, and diffraction. It became generally accepted that light acted as an electromagnetic wave.

Hertz, however, had also discovered the photoelectric effect , by which certain metals would produce an electrical potential when exposed to light. As scientists studied the photoelectric effect, it became clear that a wave theory could not account for this behavior; in fact, the effect seemed to indicate the presence of particles. For the first time in more than a century there was new support for Newton's corpuscular theory of light.

The photoelectric effect was explained by Albert Einstein in 1905 using the principles of quantum physics developed by Max Planck. Einstein claimed that light was quantized—that is, it appeared in "bundles" of energy . While these bundles traveled in waves, certain reactions (like the photoelectric effect) revealed their particulate nature. This theory was further supported in 1923 by Arthur Holly Compton, who showed that the bundles of light—which he called photons—would sometimes strike electrons during scattering, causing their wavelengths to change.

By employing the quantum theories of Planck and Einstein, Compton was able to describe light as both a particle and a wave, depending upon the way it was tested. While this may seem paradoxical, it remains an acceptable model for explaining the phenomena associated with light and is the dominant theory of our time.

See also Photon.

Resources

books

Born, Max, and Emil Wolf. Principles of Optics. New York: Pergamon Press, 1980.

Hecht, Eugene. Optics. Reading, MA: Addison-Wesley Publishing Company, 1987.